Conventional audio apparatus have only one output port. An audio jack (e.g., an audio plug) configured for receiving a single-ended signal is inserted into the output port (e.g., an audio socket). The audio jack typically has three terminals, and the three terminals receive, through the output port, a single-ended left (L) signal, a single-ended right (R) signal, and a ground voltage, respectively.
Recently, audio apparatuses with additional output ports have been made due to various demands of consumers. For example, an audio jack for receiving a differential signal has four terminals. The four terminals receive differential L signals and differential R signals through an output port.